In the manufacture of substrates, for example, printed circuit boards (“PCB”), it is frequently necessary to apply small amounts of viscous fluids, i.e. those with a viscosity greater than fifty centipoise. Such fluids include, by way of example, general purpose adhesives, solder paste, solder flux, solder mask, grease, oil, encapsulants, potting compounds, epoxies, die attach pastes, silicones, RTV and cyanoacrylates.
During manufacture, a PCB is often delivered to a viscous fluid dispenser that is mounted to a gantry system and thus movable with three axes of motion above the PCB, for example in a standard X-Y-Z Cartesian coordinate system. The moving fluid dispenser operates in combination with a camera, also mounted to the gantry system and fixed relative to the fluid dispenser, and is capable of dispensing dots of viscous fluid at desired locations on the PCB.
As PCB's are becoming denser and the components mounted thereon are becoming smaller, it is increasingly critical that the dots of viscous fluid are dispensed with a high degree of accuracy, referred to generally as “placement accuracy” or “positional accuracy.” One method for evaluating placement accuracy is by comparing the location at which a dot was intended to be dispensed with the location at which the dot was actually dispensed. Many factors contribute to poor placement accuracy of dispensed dots of viscous fluid, including poor camera calibration, misalignment or slight shifting of physical components in the fluid dispensing system, internal software errors, and human error associated with data entered into the software programs controlling the fluid dispenser or camera systems.
A critical step in operating a fluid dispensing system is calculating a camera-to-needle offset value to account for the difference in positions of the fluid dispenser and the camera operating therewith relative to the X-Y plane along which the fluid dispenser and camera move. “Camera-to-needle offset” refers to the distance between the center of the fluid dispenser nozzle, or needle, from which fluid is dispensed and the center of the camera image sensor used to identify the locations at which fluid is dispensed. This camera-to-needle offset value is accounted for by the computer operating the fluid dispensing system so that fluid is then properly dispensed at locations identified by the camera in view of its offset from the dispensing nozzle. For example, a fluid dispensing nozzle and a camera image sensor may be mounted to the gantry system such that their centers are separated by a distance of fifteen centimeters. This fifteen centimeters is recorded as the camera-to-needle offset value, which is then accounted for by the fluid dispensing system when calculating the locations at which to dispense dots of fluid after the locations have been identified by the camera.
Although a preliminary calculation of camera-to-needle offset is performed when the fluid dispenser is installed, it is often insufficient to yield fluid dots that are dispensed with high placement accuracy. In other words, there is still a measurable distance between the location at which a dot was intended to be dispensed, and the location at which the dot was actually dispensed. This is due at least in part to additional sources of error not accounted for by the preliminary camera-to-needle offset value. Such additional sources of error may include slight mechanical shifting of system components after initial movements, which may render the preliminary offset value inaccurate, and/or slight inaccuracies in the execution of software controlling the fluid dispensing system. Current calibration methods for fluid dispensing systems fail to provide steps that adequately account for these additional sources of error so as to optimize dot placement accuracy.
There is a need, therefore, for a fluid dispensing system calibration method that addresses these shortcomings.